Conventionally, for improvement in visibility, liquid crystal displays or the like generally use techniques for condensing emitted light into the front direction and enhancing brightness with a surface-shaped, condensing element such as a prism sheet and a lens array sheet so that the light emitted from light sources can efficiently enter the liquid crystal displays or the like.
Because of the mechanism, however, such a surface-shaped, condensing element needs a large difference in refractive index for condensation of light and thus needs to be placed through an air layer. Thus, the condensing element has problems of an increased number of components, optical loss due to unnecessary scattering, and visible surface defect and visible contamination of setting gap with foreign matter.
It is proposed that for improvement in the visibility of liquid crystal displays or the like, techniques for increasing the emission brightness of polarized light should include a lighting system comprising a reflective layer provided on the underside of a light guide plate and a reflective polarizer provided on the emission side for the purpose of allowing light from the light source to efficiently enter the liquid crystal displays or the like. The reflective polarizer has the function of separating the components of incident natural light into transmitted polarized light and reflected polarized light depending on polarization state. The reflective polarizer is broadly classified into a linear polarization type reflective polarizer for separating linearly polarized light and a circular polarization type reflective polarizer for separating circularly polarized light.
There is proposed an optical element comprising reflective polarizers and an element (a retardation layer) for changing polarization state sandwiched between the reflective polarizers. Linear polarization type reflective polarizers use Brewster angle to separate polarized light, and those using a vapor-deposited bandpass filter are known (for example, see German Patent Application Laid-Open No. 3836955). Circular polarization type reflective polarizers use Bragg reflection, and for example, those using the selective reflection properties of cholesteric liquid crystals are known (for example, see Japanese Patent Application Laid-Open (JP-A) No. 02-158289, JP-A No. 06-235900, and JP-A No. 10-321025).
Optical devices using the reflective polarizer have an angle-dependency with respect to the transmittance and the reflectance and can condense diffused light into the front direction. In addition, the reflectance of these reflective polarizing elements varies with incident angle, and thus if they are appropriately optically designed, they can transmit light only in the front direction. On the other hand, the non-transmitted light is reflected and allowed to return toward the light source without being absorbed so that it can be recycled to achieve efficient condensing properties. In the collimating system using the reflective polarizer, the degree of parallelization can be designed to be high, and lights can be condensed and collimated into a narrow range of at most ±20 degrees from the front direction. It is difficult to achieve such a level in a conventional simple backlight system using a prism sheet or a microdot array.
In such optical elements (collimating films), however, the shielding rate is not satisfactory, and residual transmitted lights are observed with respect to obliquely incident lights. If the width of the wavelength band to be shielded is narrow, secondary transmission can occur in oblique directions, which can lead to release and waste in oblique directions and can lead to variations in transmittance depending on wavelength so that a problem of coloring or the like can be caused in some cases.